This invention relates to thermal barrier coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a method for forming thermal barrier coating having relatively low densities and low thermal conductivities.
Thermal barrier coatings (TBC) have found wide use for thermally insulating the exterior surfaces of high-temperature gas turbine components in order to minimize the service temperatures of such components. Various ceramic materials have been employed as the TBC material, particularly zirconia (ZrO2) stabilized by yttria (Y2O3), magnesia (MgO) or other oxides. These particular materials are preferred because they exhibit suitable thermal properties and can be readily deposited by plasma spray, flame spray and vapor deposition techniques. An example of the latter is electron beam physical vapor deposition (EBPVD), which produces a thermal barrier coating having a columnar grain structure that is able to expand with its underlying substrate during thermal cycling without causing damaging stresses. As a result, TBCs deposited by EBPVD generally exhibit enhanced strain tolerance, and are therefore relatively resistant to spalling.
To be effective, thermal barrier coatings must have low thermal conductivity, be capable of being strongly adhered to the article, and remain adherent through many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials used to form turbine engine components. For this reason, thermal barrier coatings are typically deposited on a metallic bond coat formed on the surface of the component, yielding what is termed a thermal barrier coating system. The metallic bond coat is typically a diffusion aluminide or an oxidation-resistant overlay alloy coating, such as MCrAlX where M is iron, cobalt and/or nickel and X is hafnium, zirconium, yttrium, tantalum, platinum, palladium, silicon or a combination thereof. The bond coat promotes the adhesion of the insulating layer to the component while also inhibiting oxidation of the underlying superalloy.
The process of depositing TBC as well as other ceramic coatings by EBPVD generally entails initially loading the components to be coated into a coating chamber capable of operating at elevated temperatures (e.g., at least 900xc2x0 C.) and subatmospheric pressures, typically about 0.005 mbar. For the purpose of maintaining stoichiometric proportions of the deposited ceramic, an oxygen partial pressure of up to 50% is maintained in the coating chamber by injecting oxygen and typically an inert gas into the chamber as either a mixture or separately through different inlets. The component is supported in proximity to an ingot of the ceramic coating material to be deposited, and an electron beam is projected onto the ingot so as to melt the surface of the ingot and produce a vapor of the ingot material that deposits onto the component, forming an adherent ceramic layer. The deposition process is continued until the desired thickness for the TBC is obtained. Yttria-stabilized zirconia (YSZ) having a thickness on the order of about 125 micrometers (about 0.005 inch) or more is relatively common in the art.
Though YSZ deposited by EBPVD is a highly successful coating system for protecting turbine engine components, there is an ongoing effort to improve the deposition process and thermal properties of such coatings, including reduced density and thermal conductivity.
The present invention is a method for producing a thermal barrier coating system on a component that will be subjected to a hostile thermal environment. Examples of such components include turbine, combustor and augmentor components of gas turbine engines. The thermal barrier coating system includes a ceramic thermal barrier coating (TBC), preferably yttria-stabilized zirconia (YSZ) having a columnar grain structure. According to the invention, the ceramic TBC is deposited by electron beam physical vapor deposition (EBPVD) using process parameters that significantly reduce the density and thermal conductivity of the TBC, and therefore improve its effectiveness.
The method of this invention generally entails depositing the ceramic TBC by EBPVD within a coating chamber held at an absolute pressure of greater than 0.010 mbar and with an oxygen partial pressure of greater than 50% of the absolute pressure. The desired partial pressure of oxygen can be obtained by flowing oxygen into the coating chamber at a rate that is greater than the combined flow rate of any other source gases into the coating chamber if an absolute pressure of above 0.010 mbar is maintained within the chamber. Under these conditions, the ceramic material is evaporated with an electron beam to produce a vapor of the ceramic material, which then deposits on a targeted component to form a spall-resistant layer of the ceramic material.
According to the invention, maintaining an oxygen partial pressure of greater than 50%, e.g., greater than 0.005 mbar when using an absolute coating pressure of 0.010 mbar, more preferably a partial pressure at or near 100%, has a significant effect on the density and thermal conductivity of the deposited ceramic coating. Specifically, the use of higher oxygen partial pressures has been shown to reduce both density and thermal conductivity of a ceramic coating deposited by EBPVD. As a result, thinner ceramic coatings can be deposited to obtain the same thermal insulating effect as thicker prior art coatings of the same ceramic material. Alternatively, greater temperature gradients can be achieved by depositing layers of the same thickness as prior art coatings. Another advantage observed with this invention is that higher coating deposition rates have been obtained as compared to prior art processes performed at conventional oxygen partial pressures, i.e., 50% and less, or oxygen flow rates less than that of other gases into the coating chamber. Accordingly, the method of the present invention not only improves the thermal properties of the resulting TBC, but also improves manufacturing economies.
Other objects and advantages of this invention will be better appreciated from the following detailed description.